JP-A-2005-207405 (U.S. Pat. No. 7,051,725 B2) shows a cylinder air-fuel ratio control system including the controller. In this system, air-fuel ratios of a plurality of cylinders are estimated for the respective cylinders on the basis of the output of one air-fuel ratio sensor disposed in an exhaust confluent portion where exhaust gas from the plurality of cylinders joins, air-fuel ratio correction quantities of the respective cylinders (cylinder air-fuel ratio correction quantities) for correcting variations in an air-fuel ratio between the cylinders are computed for the respective cylinders, the cylinder air-fuel ratio correction quantities are learned by smoothing processing or the like, and cylinder air-fuel ratio control for controlling the air-fuel ratios (fuel injection quantities) of the plurality of cylinders for the respective cylinders is performed on the basis of the cylinder air-fuel ratio correction quantities and their learning values. Further, in this cylinder air-fuel ratio control system, on operating condition that the estimation of the cylinder air-fuel ratio is difficult (the computation of the cylinder air-fuel ratio is difficult), the cylinder air-fuel ratio control is performed by the use of the learning values of the cylinder air-fuel ratio correction quantities.
In this cylinder air-fuel ratio control system, there may be a case where, for example, if the fuel injection valve or the like of any cylinder becomes abnormal and the control of the air-fuel ratio of the cylinder becomes difficult, variations in the air-fuel ratio between the cylinders becomes abnormally large and the cylinder air-fuel ratio correction quantity and its learning value become abnormally large. Even if the cylinder air-fuel ratio control is continuously performed in this state, variations in the air fuel ratio between the cylinders cannot be reduced, which results in excessively correcting the air fuel ratio of the normal cylinders other than the abnormal cylinder and hence raises a problem that exhaust emission deteriorates.
Moreover, in the foregoing cylinder air-fuel ratio control system, there may be a case where when the air-fuel ratio control becomes difficult for any cylinder because of the failure of the fuel injection valve or the like, the air-fuel ratio of the abnormal cylinder for which the air-fuel ratio control becomes difficult is greatly varied in a lean direction. As a result, the air-fuel ratio of exhaust gas flowing into a catalyst (in other words, exhaust gas of a mixture of exhaust gas of the abnormal cylinder and exhaust gas of the normal cylinders) can be shifted in the lean direction. If the air-fuel ratio of exhaust gas flowing into the catalyst is shifted in the lean direction, the quantity of lean component (oxygen quantity) flowing into the catalyst is increased to accelerate oxidation reaction of rich components such as HC and CO to raise a possibility that the catalyst may be overheated and failed by the reaction heat.
Moreover, to prevent a catalyst for cleaning the exhaust gas of an internal combustion engine from being failed by overheat, there is proposed an apparatus as described in JP-A-56-20727 which controls an air-fuel ratio in a direction to decrease the temperature of the catalyst when the catalyst is determined to be overheated.
When the catalyst is determined to be overheated, if the air-fuel ratio of each cylinder is controlled by the use of this apparatus in a direction to decrease the temperature of the catalyst, there are raised the following problems: because the air-fuel ratio of the abnormal cylinder for which the air-fuel ratio control is difficult cannot be controlled with high accuracy in the direction to decrease the temperature of the catalyst, it is difficult to control the air-fuel ratio of exhaust gas (in other words, exhaust gas of a mixture of exhaust gas of the abnormal cylinder and exhaust gas of normal cylinders) with high accuracy in a direction to decrease the temperature of the catalyst; and because the air-fuel ratio of each cylinder is controlled in the direction to decrease the temperature of the catalyst after the catalyst is determined to be overheated, it is impossible to prevent the catalyst from being overheated before the catalyst is overheated.